As an application requiring high strength and low thermal expansion properties at high temperatures, there are known, for example, ring-shaped components for use as blade rings or seal ring retainers of gas turbines. Conventionally, in ring-shaped components for use as blade rings of gas turbines, and the like, high strength and low thermal expansion properties have been required even at high temperatures. Materials used in such applications have included SCPH21 (1.2Cr-0.5Mo cast steel), SCPH32 (2.2Cr-1.0Mo cast steel), SCS1 (13Cr cast steel) and the like.
In recent years, however, it is required to reduce clearances for absorbing differential thermal expansion between blades and blade rings and between seal fins and seal ring retainers, in order to enhance the efficiency of gas turbines. Consequently, a material exhibiting lower thermal expansion than conventional materials is needed for the formation of such ring-shaped components for use as blade rings and seal ring retainers of gas turbines. As low-thermal expansion alloys meeting this requirement for low-thermal expansion properties, Invar alloy (36% Ni—Fe), Super-invar alloy (31% Ni-5% Co—Fe) and the like are known, and a large number of Invar alloy castings utilizing Invar properties have been reported.
However, in most of the Invar alloy castings, importance is usually attached to an average coefficient of thermal expansion in a relatively low temperature region extending from ordinary temperature to about 200° C. In fact, these Invar alloy castings have excellent low-thermal expansion properties in a low temperature region of the order of 200° C. However, in such applications as ring-shaped components for use as blade rings or seal ring retainers of gas turbines which are heated to a high temperature of the order of 500° C. during service, such Invar alloy castings are unsuitable because the clearances between blades and blade rings and between seal fins and seal ring retainers change considerably as a result of a rapid increase in coefficient of thermal expansion. Moreover, owing to its low strength, Invar alloy cannot be used in applications requiring both a low coefficient of thermal expansion and high strength, such as ring-shaped components for use as blade rings and seal ring retainers of gas turbines.
In order to maintain low thermal expansion up to a high temperature region of the order of 500° C., it is necessary to shift the magnetic transformation point to a higher temperature. As a means to this end, an increase in Ni content and the addition or increase of Co is commonly known. Such high-Ni/Co Invar alloy castings have been proposed in Japanese Patent Laid-Open No. 41350/'82, Japanese Patent Laid-Open No. 21037/'89, and Japanese Patent Laid-Open No. 60255/'88. In the alloy casting described in the aforementioned Japanese Patent Laid-Open No. 41350/'82, the combined content of Ni and Co is in the range of 38 to 45%. As a result, it is described therein that its coefficient of thermal expansion in a temperature range extending from ordinary temperature to 300-500° C. is reduced and, moreover, its ordinary-temperature strength is very high. This alloy casting can surely exhibit low-thermal expansion properties in a low temperature region of the order of 300° C. However, in high-temperature applications such as ring-shaped components for use as blade rings or seal ring retainers of gas turbines, its oxidation resistance and high-temperature strength at about 500° C. are unsatisfactory because of a low Cr content up to 1.0%. Moreover, in this alloy casting, no consideration is given to Si that is important for the improvement of castability or to Mg and S that are necessary for the purpose of inoculation for graphite.
In the alloy described in Japanese Patent Laid-Open No. 21037/'89, the Ni content is as low as 28.0-32.0%, but a large amount of Co is added in the range of 8.0-18.0%. Thus, it is disclosed that its average coefficient of thermal expansion in a temperature range of 30° C. to 500° C. shows a low value of not greater than 7.5×10−6/° C. However, this alloy does not contain any element that serves to improve high-temperature strength and oxidation resistance at 500° C., and is hence unable to achieve high strength at a high temperature of the order of 500° C.
The alloy described in Japanese Patent Laid-Open No. 60255/'88 contains 29-33% of Ni and 4.5-6.5% of Co. However, owing to a low Ni content, its average coefficient of thermal expansion up to a high temperature of the order of 500° C. is unsatisfactorily high. Moreover, 1.0 to 2.7% of C is added in order to improve machinability with importance attached to machining accuracy, so that a large amount of spheroidal graphite is precipitated. Not only the precipitation of a large amount of spheroidal graphite causes a reduction in strength on the other hand, but also the addition of a large amount of C increases the coefficient of thermal expansion up to a high temperature (500° C.).